programmable interval timer 8254

PROGRAMMABLE

INTERVAL TIMER 8254

Dr. Sikder Sunbeam Islam

EEE, IIUC

ABOUT 8254

The 8254 programmable interval timer/counter is

functionally similar to the software designed

counters and timers. It generate accurate time

delay. 8254 is the high speed version of the 8253.

Features:

3-identical 16-bit counters.

Six modes of operation.

Single +5v power supply.

24-pin DIP(Dual in-line package).

Can count either in BCD or Binary.

Fig.1 2

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APPLICATIONS

Real time clock

Event-counter

Digital one-shot

Programmable rate generator

Square wave generator

Complex waveform generator

Complex motor controller

Used to refresh DRAM memory.

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INTERNAL BLOCK DIAGRAM OF 8254

Fig.2: Internal blocks of 8254

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COUNTER SELECTION

CLK : The clock input is the timing source for each of the internal

counters.

OUT: A counter output.

GATE : The gate input controls the operation of the counter for a

mode.

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INTERNAL BLOCK DIAGRAM OF A

COUNTER

Fig.3: Internal blocks of counter

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INTERNAL BLOCK DIAGRAM OF A COUNTER (CONTINUES.)

The Counters are fully independent. Each Counter may operate in a different Mode.

The Control Word Register is shown in the figure; it is not part of the Counter itself, but its contents determine how the Counter operates.

The status register, shown in Fig.3, when latched, contains the current contents of the Control Word Register and status of the output.

The actual counter is labeled CE (for „„Counting Element‟‟). It is a 16-bit presettable synchronous down counter.

OL(M) and OL(L) are two 8-bit latches. OL stands for „„Output Latch‟‟; the subscripts M and L stand for „„Most significant byte‟‟ and „„Least significant byte‟. These latches normally „„follow‟‟ CE.

There are two 8-bit registers called CR(M) and CR(L), CR for „„Count Register‟‟. Both are normally referred to as one unit and called just CR. When a new count is written to the Counter, the count is stored in the CR and later transferred to the CE. 7

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CONTROL WORD FORMAT

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8254 OPERATION

8254 Write operation

The programming procedure for the 8254 is very flexible. Only two conversion need to be remember.

For each Counter, write the Control Word in the control register before the initial count is written.

Load the low-order byte of a count in the counter register (CR).

Load the higher-order byte of a count in the counter register (CR).

The initial count must follow the count format specified in the Control Word (least significant byte only, most significant byte only, or least significant byte and then most significant byte).

With a clock and an appropriate gate signal to one of the counters, the above steps should start the counter and provide appropriate output according to the control word. 9

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8254 READ OPERATIONS

There are three possible methods for reading the

counters:

A simple read operation (read counter after

stopping)

The Counter Latch Command (reading on the

fly), and

The Read-Back Command (read multiple counter

at a time).

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A SIMPLE READ OPERATION

This operation read the counter after stopping.

Counting is stopped by controlling the gate input or the

cloak input of the selected counter.

Then two I/O read operation are performed by the MPU:

1. The first I/O operation reads the low order byte.

2. The second I/O operation reads high order byte.

Counter Latch Command

This allows reading the contents of the Counters “on the fly''

without affecting counting in progress.

The selected Counter's output latch (OL) latches the count at

the time the Counter Latch Command is received.

This count is held in the latch until it is read by the CPU (or

until the Counter is reprogrammed). The count is then

unlatched automatically and the OL returns to “following''

the counting element (CE). 11

[Latches can be memory devices, and can store one bit of data for as long as the device is powered.]

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COUNTER LATCH COMMAND (CONTINUED)

Fig: Counter Latching Command Format

Example:

;Latching Counter-1

MOV AL, 01000000B ; count

latched for counter 1.

OUT CNT_REG, AL;

;Reading Counter-1

MOV DX, COUNTR1

IN AL, DX

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If the counter is latched, and then latched again before the count is read,

the second Counter Latch Command is ignored.

[“;” is used to notify statements

in program]

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READ-BACK COMMAND

This command is used to read several counters at a time. It

eliminates the need of writing separate counter-latch commands

for different counters.

It allows the user to check the count value, programmed Mode,

and current states of the OUT pin and Null Count flag of the

selected counter/ counters.

The command can be written into the Control Word Register and

has the format shown in Figure.

Figure: Read-back command format

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READ-BACK COMMAND (CONTINUED)

By setting the COUNT bit D5 =0, multiple counter output

latches (OL) can be latched (counted).

Each counter's latched count is held in the OL until it is read

(or reprogrammed). The counter is automatically unlatched

when it is read.

By setting STATUS bit D4 = 0; status information of selected

counter(s) can be latched. The counter status format is shown

in Figure below.

Bits D5 through D0 shows the

counter's programmed Mode.

OUTPUT bit D7 contains the

current state of the OUT pin.

This allows the user to

monitor the counter's output

via software. Figure: Counter status format 14

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Example:

;Read-back Command for Count 0

MOV AL, 11000010B ; count latched for Count 0

OUT CNTRL_REG, AL;

;Reading the latched status for count 0

MOV DX, COUNTR0

IN AL, DX ; Reading Status

MOV AH, AL

;Reading the latched count for Counter 0

IN AL, DX ; Reading LSB of Counter 0

MOV BL, AL

IN AL, DX ; Reading MSB of Counter 0

MOV BH, AL

READ-BACK COMMAND (CONTINUED)

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If both count and status of a counter are latched, the first read operation of

that counter will return latched status. The next one or two reads return

latched count.

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MODES OF OPERATION

Mode 0: Interrupt on terminal count.

Mode 1: Hardware Retriggerable One-Shot.

Mode 2: Rate Generator.

Mode 3: Square Wave Mode.

Mode 4: Software Triggered Mode.

Mode 5: Hardware Triggered Mode.

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MODE 0: INTERRUPT ON TERMINAL COUNT OR

EVENT COUNTER

Figure: Suppose count 7 is loaded. 17

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MODE 0: INTERRUPT ON TERMINAL COUNT

OR EVENT COUNTER(CONTINUES).

Once the count is loaded in the register, the

counter is decremented. When the count reaches

to zero, the OUT goes High. Count starts from N.

The OUT remains High until the new count or

command is loaded.

The Gate (G) input must be a Logic 1 to allow the

counter to count.

If G becomes Logic 0 in the middle of the count,

the counter will remain stop until G again

becomes a Logic 1.

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MODE 1: HARDWARE RETRIGGER ABLE ONE-SHOT:

MONOSTABLE MULTIVIBRATOR.

OUT is initially High.

When Gate is triggered, OUT goes to Low.

On completion of count, OUT goes to High again.

If the Gate input occurs within the duration of

counting, the counter is again reloaded with

count and start counting from the beginning.

Figure: Suppose count 5 is loaded.

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MODE 2: RATE GENERATOR:

When count N is loaded, the OUT remains High(1) for

N-1 clock period and Low (0) for 1 clock period.

The count N is reloaded automatically and the pulse is

generated continuously.

Count N=1, invalid.

When the Gate is logic zero, there will be no counting.

When the Gate is logic 1, then normal counting is

started.


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MODE-3 :SQUARE WAVE GENERATOR

Generate continuous Square wave at the out put.

Gate should be maintained at logic 1 always.

When count N loaded, if N is EVEN then OUT

will be High for N/2 clock cycles and Low for N/2

clock cycles.

If N is ODD, output remains HIGH for (N+1) /2

and low for (N-1) /2.


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MODE 4: SOFTWARE TRIGGERED STROBE

After mode is set, the output goes high.

If count of N is loaded, then OUT will be high for N clock cycles and low for one clock cycle at the end.

The cycle does not begin until the counter is loaded again.

Gate input must be maintained High during operation.


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MODE 5: HARDWARE TRIGGERED STROBE

(RETRIGGERABLE)

Operation is similar to Mode-4, except that it is started

by a trigger pulse on the G pin instead of by software.

When the GATE pulse is triggered from low to high the

count begins. At the end of the count OUT goes Low for

one clock period.


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PROBLEM 1

a. Identify the port address of the control register and counter 2 in

Figure-1.

b. Write a subroutine to initialize counter 2 in mode 0 with a count of

50,000.The subroutine should also include reading counts on the fly;

when count reaches zero, it should return to the main program.

c. Write a main program to display seconds by calling the subroutine as

many times as necessary.

Figure-1

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Problem 1 (Continued)

Solution (a):

According to the Figure-1,

A15 A14 A13 A12……………….A3 A2 A1 A0 Address Counter/Register

1 0 0 ……………… 0 0 0 8000H Counter0

1 0 0 ……………… 0 0 1 8001H Counter1

1 0 0 ……………… 0 1 0 8002H Counter2

1 0 0 ……………… 0 1 1 8003H Control Register

Address of Counter 2 = 8002H

Address of Control Register = 8003H

Solution (b):

We have to initialize counter 2 in Mode0.

Count=(50,000)10 = C350H

Control Word to initialize counter 2 in mode0 and to load 16-bit count:

1 0 1 1 0 0 0 0

Counter 2 Load 16 bit Count Mode0 Binary count

So, Control Word = 10 11 000 0=B0 H

1 0 0 0 0 0 0 0

Counter 2 Load 16 bit Count Don't Care

So, Control Word = 10 00 000 0=80H (Counter Latch Command)

To read count 2 on the fly counter latch command is:

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Solution (b).

Subroutine:

COUNTER PROC NEAR

CNT2 EQU 8002H

CNTR EQU 8003H

MOV AL, B0H

OUT CNTR, AL

MOV AL, 50H

OUT CNTR2, AL

MOV AL, C3H

OUT CNTR2, AL

READ: MOV AL, 80H

OUT CNTR, AL

IN AL, CNT2

MOV DL, AL

IN AL, CNT2

OR AL, DL

JNZ READ

RET

COUNTER ENDP


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Solution (c)

Clock frequency, fc = 2 MHz

Time period of each clock cycle,

𝑡𝑐 =1/(2 ×106)= 5 ×10−7 𝑠𝑒𝑐 Every time subroutine is called then, 50,000 × 5 ×10−7 = 25 ݉𝑠 is counted.

To count 1 sec subroutine needed to be called, 25݉𝑠= 40 𝑡𝑖݉𝑒𝑠 = 28𝐻 𝑡𝑖݉e𝑠

Main Program:

Assuming segment registers are already initialized. MOV BL, 00H

SECOND: MOV CL, 28H

WAIT: CALL COUNTER

DEC CL

JNZ WAIT

MOV AL, BL

ADD AL, 01

DAA

OUT 25H, AL ; assuming 8 bit port 25H

MOV BL, AL

JMP SECOND

HLT


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Problem 2:

Write instructions to generate a pulse in every 50 us later from counter 0.

Consider the figure of problem1.

Solution :

To generate a pulse in every 50 us later, we should initialize counter0 in

mode 2. Gate0 should be high.

Count: Clock frequency, fc = 2 MHz

Needed count, 𝑁 =𝑢݈𝑠𝑒 𝑡𝑖݉𝑒(𝑡𝑝)

𝑐݈𝑐݇ 𝑒ݎ𝑖݀(𝑡𝑐) =

50 ×10−6

5 ×10−7 = 100 = 64𝐻

Control Word:

Instructions:

CNT0 EQU 8000H

CNTR EQU 8003H

MOV AL, 14H

OUT CNTR, AL

MOV AL, 64H

OUT CNT0, AL 28

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Problem 3:

Write instructions to generate a 1 KHz square wave from Counter1. Assume

the gate of Counter1 is tied to +5V through a 10K resistor. Explain the

significance of connecting the gate to +5V. (use Figure of problem 1)

Control Word:

Solution:

To generate a square wave from Counter1, it should be initialize in Mode 3.

As, Clock time period=5 ×10−7s and time period of Square Wave=(1/1K)=10−3s

Needed count = 𝑁 =𝑢݈𝑠𝑒 𝑡𝑖݉𝑒(𝑡𝑝)

𝑐݈𝑐݇ 𝑒ݎ𝑖݀(𝑡𝑐)=

10−3

5 ×10−7=2000 = 07𝐷0𝐻

Instructions:

CNT1 EQU 8001H

CNTR EQU 8003H

MOV AL, 76H

OUT CNTR, AL

MOV AL, D0H

OUT CNT1, AL

MOV AL, 07H

OUT CNT1, AL

Gate should be maintained at logic 1 (+5V) always

as GATE =1 enables counting and GATE =0

disables counting. 29

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Problem 4:

Write a subroutine to generate an interrupt every 1sec. Consider the figure of

problem1.

Solution

To obtain a pulse every 1 sec, the count should be 2×106, which is too large

for

one 16-bit counter. We can divide this counter as follows:

Counter 1 [Clock pulse=2 MHz]--- Mode 2 ----- Count 50,000 (C350H)

Counter 2 [Clock pulse=Out1 (output of counter 1)]---- Mode 2 ---- Count 40 (28H)

So, finally we get 50,000 × 40 = 2×106 count and from OUT2 we get the

desired output.

0 1 1 1 0 1 0 0 74H

Binery count CW

Binery count CW

1 0 0 1 0 1 0 0 94H

Counter 2 LSB Only Mode2

Counter 1 Load 16 bit Count Mode2

Control word:

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Subroutine:

INTRP PROC NEAR

CNT1 EQU 8001 H

CNT2 EQU 8002H

CNTR EQU 8003H

MOV AL, 74H

OUT CNTR, AL

MOV AL, 94H

OUT CNTR, AL

MOV AL, 50H

OUT CNT1, AL

MOV AL, C3H

OUT CNT1, AL

MOV AL, 28H

OUT CNT2, AL

RET

INTRP ENDP

Problem 4:

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REFERENCE

Microprocessor and Interfacing Programming

and Hardware; Douglas V Hall, 2nd Edition.

Microprocessors and Microcomputer-Based

System Design; Mohammed Rafiquzzaman,

revised edition.

Microprocessor & Interfacing Lecture Materials;

Mohammed Abdul Kader, EEE, IIUC.

Mohammed Amer Arafah, King Saud University;

https://fac.ksu.edu.sa/sites/default/files/part8_825

4.pdf

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programmable interval timer 8254

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